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Louis Vinke, Sam Ling; Luminance modulates the contrast response in human visual cortex. Journal of Vision 2017;17(10):124. doi: https://doi.org/10.1167/17.10.124.
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© ARVO (1962-2015); The Authors (2016-present)
The vast majority of models in vision downplay the importance of overall luminance in the neural coding of visual signals, placing emphasis instead on the coding of features such as relative contrast. Given that the visual system is tasked with encoding surfaces and objects in scenes, which often vary independently in luminance and contrast, it seems plausible that luminance information is indeed encoded and plays an influential role in visuocortical processing. However, the cortical response properties that support luminance encoding remain poorly understood. In this study, we investigate the interaction between contrast response and luminance in human visual cortex, using fMRI. We assessed BOLD responses in early visual cortex (V1-V3) while participants viewed checkerboard stimuli that varied in contrast and luminance. Specifically, we utilized an adaptation paradigm that allowed us to reliably measure contrast responses at multiple spatial scales (voxel-wise and retinotopic), and across a set of luminance levels. To control for changes in pupil diameter with varying luminance levels, stimuli were viewed monocularly through an artificial pupil. We found that the extent to which the overall luminance of a signal modulates responses in visual cortex is contrast dependent, with reliable increases in contrast responses along with increasing luminance levels, but only occurring at low levels of contrast. Furthermore, the modulation strength of luminance on contrast responses did not exhibit any retinotopic bias. These results reveal that the visuocortical neural code does indeed retain and utilize information about the luminance of a visual signal, but appears to preferentially modulate the response only at low-to-zero contrast levels. This finding suggests that luminance likely plays a dominant role in visual tasks such as our perceptual encoding and segregation of surfaces.
Meeting abstract presented at VSS 2017
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